Nanostructured Metal Organic Framework-74 Derivatives for the Esterification of Levulinic Acid to Butyl Levulinate

Abstract

In this work, 12 metal–organic framework-74 (MOF-74) candidates were evaluated as heterogeneous catalysts for synthesizing the eco-friendly biofuel additive, butyl levulinate (BL). This study focuses on optimizing the catalytic performance of such frameworks by exploring the relationships between the synthesis conditions, structural composition, and catalytic properties. A range of MOF-74 derivatives with different metal centers (e.g., Zn, Cu, Mn, Ni, Co, and Mg) were solvothermally synthesized and fully characterized using PXRD, TGA, SEM, XPS, IR and BET surface area measurements. The catalytic performance of the studied MOFs in the esterification of levulinic acid (LA) to butyl levulinate (BL) was investigated, with Zn-MOF-74 emerging as the most effective catalyst, achieving a conversion rate of 96% at a LA-to-butanol molar ratio of 1:7. Catalyst loading and reaction temperature were assessed on Zn-MOF-74, whereas the most favorable conditions were found to be 120 °C and 5 wt % catalyst loading with respect to the initial mass of LA. Furthermore, the recyclability test showed a sustained crystallinity and catalytic efficiency of the Zn-based catalyst with a conversion of 91% after five cycles. Additionally, nanoscaled version of the MOF catalysts, including mixed-metal samples synthesized at room temperature, were evaluated, and showed comparable conversion rates to the solvothermally produced MOFs. Zn-MOF-74 was further modified by varying the amounts of magnetite (Fe₃O₄) nanoparticles to create magnetic framework composites (MFCs) with enhanced magnetic separation capabilities. These MFCs demonstrated catalytic conversions close to 94%, confirming that Fe₃O₄ loading did not impede the intrinsic active sites within this framework. Finally, two kinetic models were developed to analyze the reaction parameters for the esterification process. This work underscores the potential of tailored MOF structures as efficient catalysts in producing biofuel-relevant esters, offering insights for industrial biofuel applications.